<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <script src="./jquery-1.12.4.min.js"></script>
    <title>Document</title>
</head>

<body>
    <div>
        <input type="number" class="first">
        <select class="selector">
            <option>請選擇</option>
            <option>+</option>
            <option>-</option>
            <option>*</option>
            <option>/</option>
        </select>
        <input type="number" class="second">
        <button class="run">=</button>
        <input type="number" class="result">
    </div>
    <script>
        $('.run').click(() => {
            let first = parseInt($('.first').val());
            let selectFun = $('.selector').val();
            let second = parseInt($('.second').val());
            let result = eval(first + selectFun + second);
            $('.result').val(result);
        })
    </script>

</body>

</html>